In radio systems, for example in the mobile communication field, it is often desirable to use only one common antenna for transmitted and received signals. Transmitted and received signals use different frequency ranges. The antenna which is used must be suitable for transmitting and receiving in both frequency ranges. To separate the transmitted and received signals, suitable frequency filtering, which ensures that on the one hand the transmitted signals are passed on from the transmitter only to the antenna (and not in the direction of the receiver), and on the other hand the received signals are passed on from the antenna only to the receiver, is necessary.
For this purpose, a pair of high frequency filters can be used, both of them letting through a specified (i.e. the desired) frequency band (band pass filter), or a pair of high frequency filters, which both block a specified (i.e. the not desired) frequency band (band stop filter), or a pair of high frequency filters, consisting of one filter which lets through frequencies below a frequency between the transmission and reception bands and blocks those above it (low pass filter), and a filter which blocks frequencies below this frequency between the transmission and reception bands and lets through those above it (high pass filter). Other combinations of the above-mentioned filter types can also be used.
High frequency filters of the described type can be differently structured. A known high pass filter can consist of a hole or a channel in a milled or cast housing, inner conductor sections being arranged in the channel or hole and connected galvanically via so-called stubs to the outer conductor. The inner conductor sections (if the whole arrangement is to have a compact size) usually have interruptions of very small dimensions, so that the corresponding inner conductor sections are capacitively coupled on their faces. The size of the capacitive couplings between the wire sections is inversely proportional to the change of distance. The face-side capacitive coupling between the inner conductors rises further with increasing cross-section surface of the wires and increasing dielectric constant of the material which can be in the gap between the wires. Since, in the case of coaxial high pass filters which are known in the prior art and in corresponding form, relatively high capacitances are usually necessary, the gap between the faces of the inner conductor sections, which are positioned in axial extension to each other (if, as mentioned, comparatively compact outer dimensions are to be maintained), is usually less than 0.5 mm (e.g. when installed in a base station or other antenna facility). The gap is often around 0.1 to 0.2 mm.
On the basis of FIG. 12a, a corresponding coaxial high pass filter is shown in schematic axial longitudinal section (e.g. in plan view, without showing the cover which seals the outer conductor), and in FIG. 12b it is shown in axial cross-section (with a cover which seals the outer conductor), as it is known from the prior art. Differently from this form, the housing can also be divided into two or more parts, e.g. include two housing sections or housing halves which can be joined together. Similarly, the outer conductor housing can also be completely closed, so that the inner conductor arrangement is only pushed axially into this outer conductor housing. In this respect there are no restrictions.
From this it can be seen that such a coaxial high pass filter includes an outer conductor 1, which—as mentioned—usually consists of a milled or cast housing (metal, metal alloy), in which an axial hole or axial channel 3 is formed. Along this hole or channel 3, an inner conductor arrangement 5, consisting of multiple inner conductor sections 5a, is then provided. The inner conductor sections end with their inner conductor faces 5b at a short distance A, so that between the inner conductor faces 5b and thus the inner conductor sections 5a the result is a capacitive coupling. Also, for example, between these inner conductor faces 5b a dielectric D can be inserted.
The individual inner conductor sections 5a are galvanically coupled (usually centrally) to the outer conductor 1 via a branch wire 7 which runs transversely or perpendicularly to the associated inner conductor section 5a, the corresponding branch wires 7 running in lateral branch wire channels 9 (i.e. branch wire recesses 9) in the material of the outer conductor 1, and being connected galvanically to the branch wire channel floor 9a with the above-mentioned outer conductor 1 (the outer conductor 1 virtually representing the housing of the thus formed high pass filter).
Such a high pass filter in coaxial structure is to be taken as known, for example through Matthei, Young, Jones: “Microwave Filters, Impedance-Matching Networks, and Coupling Structures”, McGraw-Hill Book Company 2001, namely on page 414 (FIG. 7.07-3).
On the basis of FIG. 12c, a corresponding equivalent circuit diagram for the high frequency filter which is known according to the prior art and FIGS. 12a and 12b is reproduced. From it, it can be seen that a single inner conductor 5 is provided with individual inner conductor sections 5a, a capacitance C1 being formed between two inner conductor sections 5a, and the branch wire 7, running from the inner conductor sections 5a, which are continuous in themselves, to earth or the outer conductor 1, being connected in the form of an inductor I.
By the paired capacitive coupling of multiple wire sections or wire parts (in which the coupling can take place via a dielectric consisting of air or another material) and its galvanic connection to the outer conductor, the desired response behaviour of the thus formed high pass filter is generated. The extent of the capacitive coupling is determined by the size of the two opposite faces of the inner conductor sections which are coupled via them, by the distance A between the two face-side inner conductor sections, and the dielectric which is used between the two face-side inner conductor sections.
A comparable solution to the prior art corresponding to the representation according to FIGS. 12a and 12b has also become known from US 2009/0153270 A1, which corresponds to DE 10 2007 061 413 A1. A high pass filter with an inner conductor which comprises individual inner conductor sections is shown. Two successive inner conductor sections in axial extension to each other are arranged at a distance from each other, the faces facing each other and a subsequent inner conductor section, in a partial length, dipping into a tubular intermediate piece, which in the centre, between the two faces of the successive inner conductor sections, has a closed wall section. In this way, in the signal direction, a first coupling between the inner conductor end section, which dips into the tubular intermediate section, and the thus formed first tubular capacitor is generated, a second tubular capacitor being formed at the opposite end of the tubular intermediate piece between the tubular jacket section and the end section, which dips in there, of the nearest adjacent inner conductor section. At the face-side boundaries of the tubular intermediate piece, a spiral wire section then runs from the inner conductor to the outer conductor, so that coils are formed.
The result of this construction is an inner conductor section with, in contrast to the embodiment according to FIG. 12a, an inserted and successive doubled capacitive coupling from the end of one inner conductor section to the tubular intermediate piece and from the tubular intermediate piece to the next inner conductor section.
However, with increasing requirements for the blocking characteristics of high pass filters, multiple such inner conductor sections must be connected one behind the other to generate corresponding stop band attenuation.
The disadvantage of the high pass filters which have become known until now in corresponding coaxial structure is that correspondingly many wire sections must be arranged one behind the other to be able to implement the corresponding requirements for high pass filters, above all in the field of mobile communications. As mentioned, very small gaps must be maintained between the wire pieces to ensure sufficiently high capacitive couplings. The result of this is that the tolerance sensitivity of the structures is very high.
In contrast, it is the object of the present invention to create an improved high frequency filter (a so-called high pass filter) which, with a preferably more compact design, makes it possible to steepen the stop band.
It can and must be called quite surprising that compared with the prior art, a clearly improved high pass filter, which makes improved electrical properties and space-saving construction possible, is achievable within the invention. Additionally, the high pass filter according to the invention is distinguished by clearly improved tolerance sensitivity compared with the prior art.
The high pass filter according to the invention can also be used as a single filter, but also connected to one or more similar or different high frequency filters. The result, as a favourable application case, is also the use of the high frequency (HF) filter according to the invention in mobile communications, and there in particular in duplex filters, which—as explained above—are required in order to separate the transmitted signals which are fed into an antenna from the received signals which are received via the same antenna, and which are transmitted or received in offset frequency ranges.
The solution according to the invention consists substantially of fitting an additional inner conductor coupling element into the high frequency filter track, this additional inner conductor coupling element either being metallic and thus electrically conductive, or consisting of a metallically and/or electrically conductively coated dielectric, or including the latter. The additionally applied inner conductor coupling element according to the invention is provided in the region of the face-side coupling of the inner conductor sections. If this inner conductor coupling element is in the form of a hollow cylinder, for example, or in general provided with an inner recess, in this inner conductor coupling element the ends of the adjacent inner conductor sections, i.e. the relevant inner conductor faces, can be fully or at least partly opposite each other within the inner conductor coupling element. However, it is also possible that the inner conductor coupling element is arranged overlapping with the inner conductor sections which work with it only in a partial peripheral region, thus for example overlaps only over an axial length of the relevant face of the inner conductor section with the end region of the associated inner conductor section, in order to achieve the additional coupling here.
Also, in contrast to the prior art, the inner conductors are connected electrically to the outer conductor not by the inner conductor sections, but by corresponding branch wires from the inner conductor coupling elements.
Within the invention, by constructing a high pass filter which is structured in this way, a series of surprising advantages can be achieved.
Within the invention, it is possible to generate, below the frequency pass band, blocking poles which thus contribute to considerable steepening of the filter characteristic below the frequency pass band.
With every high pass filter according to the invention, a blocking pole can be achieved by using a corresponding inner conductor coupling element. In other words, multiple such structures can be connected one behind the other (in series), in which case multiple additional blocking poles can be generated by corresponding tuning. For completeness only, we mention here that the high pass filter according to the invention, while generating one or more blocking poles, can also be combined with other, conventional high pass filter structures. In this respect too there are no restrictions.
Within the invention, the structure of the high frequency filter can also be significantly shortened compared with the prior art. The overall result is more compact overall dimensions.
The sensitivity of the capacitive electrical coupling is also reduced by using the inner conductor coupling element.
The invention also results in a cost advantage, since the invention means that there is only a relatively small additional expenditure for the additionally provided inner conductor coupling elements, these additional costs being less compared with the additional costs of the serial circuit of additional inner conductor sections, such as are necessary today according to the prior art.
Finally, within the invention, the mechanical stability can also be increased using the inner conductor coupling elements. Above all, this applies in the case of corresponding use of a dielectric in solid form, i.e. not in air, because in this way the inner conductor sections, the inner conductor coupling elements and/or the branch wires can also be stabilised and held.
In other words, the dielectric, which is at least partly in the inner conductor coupling element in which the inner conductor sections end, can take an additional positioning function of the inner conductor coupling element and thus also of the inner conductor sections, above all when the dielectric is provided outside the inner conductor coupling element in the corresponding receiving space (hole, channel) of the outer conductor arrangement. Also, further dielectrics for mechanical stabilisation within the structures are possible, e.g. also coated dielectrics.
The structures according to the invention make it possible to transmit high powers. The result is also—which in particular is very important in mobile communications—altogether good intermodulation behaviour.
Finally, it can and must be noted that furthermore, within the solution according to the invention, good heat dissipation via the inner conductor coupling elements and for example the galvanic coupling to the outer conductor is achieved.
A further possible improvement within the invention is that the coupling of the inner conductor structures between the inner conductor coupling elements and the outer conductor does not necessarily have to be by the corresponding branch wires being connected galvanically to the outer conductor. It is also possible that the branch wires are capacitively coupled to the outer conductor. In this case too, the fixed dielectric which may exist in the outer conductor interior can also be used for positioning and fixing the branch wires which are capacitively coupled to the outer conductor.
Summarising, therefore, it can be recorded that within the invention, a high frequency filter is created, namely a so-called high pass filter, in which, by targeted addition of a structure, also called an inner conductor coupling element below, a blocking pole below the pass band can be generated. If multiple such structures are connected in series, in this way multiple blocking poles below the pass band can be generated. Such an inner conductor coupling element can be electrically conductive, e.g. because it consists of a metal or a metallic structure, or it can be formed from or include a dielectric, which for example has an electrically conductive coating. Such a version according to the invention of one or more additional blocking poles results in a clear steepening of the stop band and a shortened design, with simultaneous tolerance insensitivity of the high pass filter compared with previous solutions. The invention can be used both as an individual filter and in connection to one or more similar or different high frequency filters. One of the main applications, in addition to the single filter, is in the use with so-called duplex or, for example, triplex filters.